Existing wireless communications networks, such as a wireless Local Area Networks (LANs), contain a multitude of wireless communication devices (e.g., cellular telephones, personal digital assistants, laptop computers) located within a relatively small geographical area and that simultaneously communicate with the same wireless access point. The devices operate on one or more Radio Frequency (RF) channels, the physical resources over which information is passed between the devices. Generally, in both analog and digital wireless communications, data is modulated by a desired RF signal of suitable frequency, thereby generating a modulated RF signal. The modulated RF signal is transmitted over the air, after being converted to a digital signal if desired, to a radio receiver on a selected RF channel.
More specifically, the RF signal is modulated with a local oscillator (LO) to produce the modulated signal. The LO can directly modulate the data from baseband (approximately 0 Hz) directly to the desired transmission frequency. Other transmitters can use multiple intermediate frequency stages to modulate the data from baseband to the desired frequency. While direct modulation is desirable due to the simplicity and reduced cost due to the number of components, it is subject to an increased amount of DC offset. Several sources of DC offset exist, including characteristic mismatches of electronic components in the different transmission paths, which provide a baseband DC offset, and coupling of the LO signal directly into the transmission path. Similar problems exist in receivers. These DC offset errors combine to provide an undesirable LO leakage signal that can interfere with detection and demodulation of the transmitted signal in the receiver.
To minimize the LO leakage signal, training systems may be incorporated into the transmitter and/or receiver. Such training systems use both digital and analog circuits that temporarily configure the transmitter/receiver to a known training state to estimate and compensate for the LO leakage signal (from both the direct LO coupling component and baseband DC offset component) and to achieve optimal post-training performance. While such techniques enjoy many advantages, they are expensive in terms of processing power and additional components. Moreover, the modulation must be stopped while such tuning occurs. This delays signal transmission and in extreme cases may lead to data loss. It would thus be desirable to tune and automatically remove the LO leakage signal on the fly without having to shut down the modulation while tuning.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments shown so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. Other elements, such as those known to one of skill in the art, may thus be present.